1. Field of the Invention
This invention relates to an improvement in a gas-discharge tube type photographic flash device.
2. Background of the Invention
Recently, automatic flash devices are constituted to have a function that amount of flashed light (i.e., product of intensity of light and time period of flashing) is automatically controlled by receiving light reflected by the photographic object.
In addition to the abovementioned automatic function, some flash devices are capable of the additional function of flashing light with a pre-selected amount of light, the selection being made by adjusting an electric device such as variable resistor.
FIG. 1 shows one example of such conventional flash device wherein the light flashes with a pre-selected amount of light. In FIG. 1, the positive and negative terminals + and - are connected to a known DC-DC converter not shown in the drawing. A main capacitor 1 is charged up by the DC-DC converter, and at the same time, a second capacitor 18 is charged up by a current flowing through the positive terminal +, a resistor 17, a capacitor 18, a zenor diode 19 and the negative terminal -. When the thyristor 5 is turned ON by means of a thyristor trigger circuit 3, and when a triggering pulse is coupled to a flash tube 4, for example, a xenon lamp, from a flash triggering circuit 2, the flash tube 4 is triggered and starts discharge the electric energy stored in the main capacitor 1. Since the flash tube 4 and the thyristor 5 becomes ON, the electric charge in the capacitor 18 is discharged through the resistor 17, the flash tube 4, the thyristor 5 and the zenor diode 19. Accordingly the zenor voltage appears across both terminals of the zenor diode. Therefore, a third capacitor 16 starts to be charged by the discharge of the second capacitor 18 through the resistor 17, the flash tube 4, thyristor 5 and a variable resistor 15. When the voltage across the terminals of the capacitor 16 reaches a specified level, a thyristor 13 becomes ON. A capacitor 12 is already charged by this time by a current through a resistor 11, the capacitor 12, a primary coil of the transformer 10, and a zenor diode 19. Accordingly, by the turning ON of the thyristor 13, the charge of the capacitor 12 is discharged through the primary coil of the transformer 10, thereby giving a pulse signal th the quenching discharge tube 9. Therefore, the quenching discharge tube 9 becomes ON thereby causing the capacitor 7 to impress an inverse voltage through the quenching discharge tube 9 on the thyristor 5 thereby turning it OFF, hence stopping the discharge current of the flash tube 4.
In the abovementioned conventional circuit, the selection of the amount of the flash light, namely the guide number (G.N.) is made by adjusting variable resistor 15. Namely, by varying the variable resistor 15, the time period of charging up of the third capacitor 16 is selected, and therefore, the time period of flashing of the tube 4, i.e., the amount of the flash light is set by the adjustment of the variable resistor 15. In such devices, when the voltage across the main capacitor 1 is lowered by a voltage drop of a power source battery of the DC-DC converter or by a sequential flashing without sufficient pause time inbetween, the voltage of the second capacitor 18 is lowered, while the zenor voltage of the zenor diode 19 is constant. As shown in FIG. 2, for a larger voltage V.sub.1 of the main capacitor 1, the intensity of the flashed light is stronger, and for a smaller voltage V.sub.2 the intensity is weaker. On the other hand, the time period of flashing is defined by the zenor voltage and the characteristic of the thyristor 13, the time period T.sub.1 from a start of the flash to the turn on of the thyristor 13 is constant as shown in FIG. 2, irrespective of the voltage of the main capacitor 1. Therefore, when the voltage of the main capacitor 1 is lowered by some reason from a designed value, the total amount of the flash light is decreased from a designed value.